Recently remarkable development has been made in mobile communications devices such as mobile phones, etc. There are high-frequency switches for switching the connection of an antenna to a transmitting circuit, and the connection of the antenna to a receiving circuit, as high-frequency parts used in mobile communications devices.
For instance, the high-frequency switch disclosed in JP 2-108301 A handles one transmitting/receiving system for EGSM900 (extended global system for mobile communications), GSM1800, PCS (personal communications service), etc., which comprises switching elements (diodes) arranged between the transmitting circuit and the antenna, and λ/4 phase lines arranged between the antenna and the receiving circuit. Because the receiving circuit side of the λ/4 line is grounded via a diode, the high-frequency switch is operated by bias current flowing through each diode as a λ/4 switch circuit for switching signal lines.
FIG. 38 is a block diagram showing one example of RF circuits for single-band mobile phones having such high-frequency switches. A mobile communications device such as a mobile phone, etc., comprising a high-frequency switch comprises a high-frequency switch circuit SW connected to an antenna ANT; a filter circuit such as a low-pass filter f1, etc. arranged between a transmitting circuit TX and the high-frequency switch circuit SW for preventing harmonic contained in a transmission signal from the transmitting circuit TX from emitting from the antenna ANT, and preventing part of a receiving signal from the antenna ANT from entering into the transmitting circuit TX; and a filter circuit such as a band pass filter f2, etc., particularly a surface acoustic wave filter (SAW filter), arranged between the antenna ANT and a receiving circuit RX for preventing part of a transmission signal from the transmission circuit TX from entering into the receiving circuit RX, and removing noise from the receiving signal from the antenna ANT.
Developed in addition to such single-band mobile phones in accordance with drastic spreading of mobile phones are multi-band mobile phones such as dual-band mobile phones, triple-band mobile phones, etc., which comprise one mobile terminal capable of handling a plurality of communications systems. While the single-band mobile phones handle only one transmitting/receiving system, the dual-band mobile phones handle two transmitting/receiving systems, and the triple-band mobile phones handle three transmitting/receiving systems. One example of RF circuit blocks for the multi-band mobile phones is shown in FIG. 36. This example is an RF circuit block of a dual-band mobile phone comprising a diplexer DP constituted by a plurality of filters, high-frequency switch circuits SW1, SW2 for switching the connection of an antenna ANT to a transmitting circuit TX and a receiving circuit RX, so that dual-direction communications can be carried out with one antenna for both transmitting and receiving.
A mobile phone of EGSM900, etc. comprises a balanced-type high-frequency part (for instance, a low-noise amplifier LNA arranged in a receiving line RX, a mixer MIX arranged downstream thereof, etc.) having two signal lines in an RF circuit, to decrease a noise figure while increasing receiving sensitivity.
When the low-noise amplifier LNA is a balanced input type, as shown in FIG. 37, a SAW filter connected to the low-noise amplifier LNA is conventionally an unbalanced type having one signal terminal, namely has the structure of an unbalanced-unbalanced-type filter, needing a balanced-to-unbalanced transforming circuit in connection to LNA.
Though devices such as high-frequency switches, SAW filters, etc. are usually designed to have a characteristic impedance of 50 Ω, the low-noise amplifier LNA has an input impedance of about 50Ω-300Ω. Accordingly, the high-frequency parts often have different characteristic impedances, needing impedance-converting circuits. Known as a circuit element having both functions of a balanced-to-unbalanced transforming circuit and an impedance-converting circuit is a balanced-to-unbalanced transformer (balun), the use of the balun inevitably increases the number of parts. In addition, impedance matching should be taken into consideration in the connection of the balanced-to-unbalanced transformer to the SAW filter, needing accessory parts for impedance matching such as capacitors, inductors, etc. This causes the problem of increasing the size of mobile phones, resulting in cost increase.
To make high-frequency parts smaller and lighter in weight, part of elements such as capacitors, inductors, etc. for constituting high-frequency switch circuits, filter circuits, etc. are contained in a laminate (multi-layer substrate) by an LTCC (low temperature co-fireable ceramics) technology, thereby providing composite parts having a plurality of circuit functions.
For instance, JP 6-197040 A discloses a high-frequency switch comprising transmission lines and capacitors integrally contained in a laminate constituted by low-temperature-sinterable dielectric ceramic sheets. Also, JP 10-32521 A discloses a small, light-weight, high-frequency switch comprising RF interstage filters (SAW filter) integrally mounted on a multi-layer substrate. Further, JP 11-225089 A discloses a high-frequency switch for multi-band mobile phones comprising a diplexer and high-frequency switches integrally contained in a laminate made of a low-temperature-sinterable dielectric ceramic material to handle two or more transmitting/receiving systems. Parts containing such high-frequency switches and other high-frequency parts as a composite structure are called high-frequency switch modules.
Though part of circuit elements such as capacitors, inductors, etc. can be contained in the laminate by the LTCC technology to have a plurality of circuit functions in a composite structure in the high-frequency switch module, it has not been easy to obtain electric characteristics on a level acceptable for practical use. For instance, when high-frequency switches are integrated with SAW filters connected to their receiving system in the laminate, impedance matching between the high-frequency switches and the SAW filters should be taken into sufficient consideration, and they should be connected such that there is small reflection loss at their connection points. If otherwise, it would be impossible to reduce the loss of a receiving signal due to mismatch in receiving lines from the high-frequency switches to the output terminals of the SAW filters. However, the composite structure of the high-frequency switches and the SAW filters increases the loss of a transmitting signal in a transmission line from the transmitting circuit to the antenna, failing to obtaining desired electric characteristics. In addition, an attempt to balance part of high-frequency circuits constituting the high-frequency switch module has never been made so far.